Valves are used in a variety of industries to control the flow of fluids. In particular, gate valves are used extensively in the oil and gas industry to control the flow of produced fluids at various stages of drilling or production. Most gate valves used in this industry comprise a valve body having a longitudinal flow bore and a transverse gate cavity that intersects the flow bore. A gate having an opening extending transversely therethrough is disposed in the gate cavity. A valve stem is provided for moving the gate between an open position, in which the gate opening is aligned with the flow bore, and a closed position, in which the gate opening is offset from the flow bore. The gate cavity of the valve body is covered by a bonnet having an axial bore through which passes the valve stem.
Such a gate valve is associated to a valve operator assembly for selectively driving the valve stem up and down in order to close and open the gate valve. A valve operator assembly generally comprises an input member to exert a rotational motion by a manual actuation by a hand-wheel or an electric or pneumatic or hydraulic actuation. The valve operator assembly further comprises a transmission mechanism to convert a rotational motion of the input member into an axial motion of the valve stem. To quickly open and close the gate valve with a minimum number of turns, the transmission mechanism may be a screw mechanism, such as a roller screw mechanism, a ball screw mechanism or an acme screw with or without gear reducer, in order to reduce the operating torque. For more details, it is possible to refer to the patent EP-B1-1 419 334.
Since such a screw mechanism is susceptible to back-drive under fluid pressure, the gate valve can be inadvertently opened or closed. Such back-driving can not only cause problems with the desired flow regulation, but can also lead to injury an operator, for example from being struck by the rotating hand-wheel.
Accordingly, a balance system is generally provided on the valve body of the gate valve to prevent a back-driving of the transmission mechanism with the force exerted by the fluid. Such system comprises a balance stem disposed on the valve body and whish is exposed to fluid pressure to offset or balance the force exerted on the gate.
However, it is necessary to modify the design of the valve body to integrate such a balance system on the gate valve. This leads to a complex structure of the gate valve. Besides, the required space for the gate valve is significantly increased.
EP-A1-1 813 506 discloses a roller screw mechanism for use in an electric power steering device. According to this prior art, a rotation in one direction can be converted to a linear motion by the roller screw mechanism with a high efficiency, while a reversed/back-driving rotation is prevented. The electric power steering device comprises a first and second electric motors to assist the rotation of the rotating elements of the roller screw mechanism, and then decreasing steering load for the driver. The electric motors further make the conversion efficiency to be 0in the reversed direction.
However, such a device requires electric motors to control the conversion between linear and rotational motions of the roller screw mechanisms. It is then desirable to control reversed rotation only by mechanical and structural design of the roller screw mechanism elements and not by external devices.